compat:inet_frag.h: Check for frag_percpu_counter_batch

Fix up the compat layer to check for frag_percpu_counter_batch and
if not present then use atomic_sub and atomic_add as per the
backport in the 3.16.50 LTS kernel.  Fixes compile errors on
3.16 series kernels from 3.16.50 on.

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Justin Pettit <jpettit@ovn.org>

datapath/linux/compat/include/net/inet_frag.h

@@ -30,6 +30,7 @@ static inline bool inet_frag_evicting(struct inet_frag_queue *q)
 #endif
 
 #ifndef HAVE_SUB_FRAG_MEM_LIMIT_ARG_STRUCT_NETNS_FRAGS
+#ifdef frag_percpu_counter_batch
 static inline void rpl_sub_frag_mem_limit(struct netns_frags *nf, int i)
 {
 	__percpu_counter_add(&nf->mem, -i, frag_percpu_counter_batch);
@@ -41,6 +42,19 @@ static inline void rpl_add_frag_mem_limit(struct netns_frags *nf, int i)
 	__percpu_counter_add(&nf->mem, i, frag_percpu_counter_batch);
 }
 #define add_frag_mem_limit rpl_add_frag_mem_limit
+#else /* !frag_percpu_counter_batch */
+static inline void rpl_sub_frag_mem_limit(struct netns_frags *nf, int i)
+{
+	atomic_sub(i, &nf->mem);
+}
+#define sub_frag_mem_limit rpl_sub_frag_mem_limit
+
+static inline void rpl_add_frag_mem_limit(struct netns_frags *nf, int i)
+{
+	atomic_add(i, &nf->mem);
+}
+#define add_frag_mem_limit rpl_add_frag_mem_limit
+#endif /* frag_percpu_counter_batch */
 #endif
 
 #ifdef HAVE_VOID_INET_FRAGS_INIT